\begin{tikzpicture}
    \tkzDefPoints{0/0/O}
    \tkzDefPoint(200:1.5){A}
    \tkzDefPoint(20:1.5){B}
    \tkzDefPoint(235:1.5){C}
    \tkzDefPoint(305:1.5){D}
    \tkzDefLine[altitude](C,A,D)  \tkzGetPoint{E}
    \tkzDefLine[altitude](C,B,D)  \tkzGetPoint{F}

    \tkzDrawCircle[thick](O,A)
    \tkzDrawSegments(A,B  A,E  B,F)
    \tkzDrawLine[add=0.1 and 0.1](E,F)
    \tkzDrawPoint(O)
    \tkzMarkRightAngle[size=.2](A,E,C)
    \tkzMarkRightAngle[size=.2](B,F,D)
    \tkzLabelPoints[above](O)
    \tkzLabelPoints[left](A)
    \tkzLabelPoints[right](B)
    \tkzLabelPoints[below](C)
    \tkzLabelPoints[below](D)
    \tkzLabelPoints[below left](E)
    \tkzLabelPoints[below right](F)
\end{tikzpicture}

